In order to enable the pressure in the interior of the skull to be measured, it has hitherto been customary to drill a hole of about 11 mm diameter in the skull bone and to cut a thread, in order to enable a pressure sensor to be adjusted at the correct depth and with the measurement membrane aligned coplanar to the dura mater (hard meninx, hereinafter called "dura"). For this purpose, a precise remeasurement of the depth to which the thread can be cut is necessary. When the thread itself is cut and also when the sensor is screw-adjusted, relatively large, especially radial forces act, and this causes the bone material to recede. The consequence is that, even if an adapter sleeve inserted into the bone by means of the thread is used, the entire measurement system starts to wobble after some time. It must then be readjusted again and again in order to meet the absolute requirement of an accurate measurement, namely, the coplanar-epidural alignment of the measurement membrane.
A further problem in the hitherto usual methods for measuring the pressure in the interior of the skull results from the extensive sticking of the adapter sleeves in the bone. When the adapter sleeves have to be removed again after a measurement period, it happens to quite frequently that a part of the bone is broken or torn out. Moreover, it is found in practice that cutting the thread in the skull bone for insertion of the adapter sleeve demands extensive experience of the brain surgeon. In spite of extensive experience, problems arise again and again due to tilting of the measuring sensor.
An improvement over the screwed-in adapter sleeve is described in published European patent application No. 0074037. According to EPA No. 74037, after the scalp has been cut open, a hole with defined steps is first drilled in the skull bone by means of a stepped trephine, into which hold the adapter sleeve is inserted, which sleeve is in the form of a flexible tubular member provided with outward-expanding flexible tongues and, in the upper region, has a widened section which protrudes outwards like a flange and is meant to ensure, by interaction with the stepping in the drilled table, the correct reference depth for the insertion of the pressure sensor. The adapter sleeve of EPA No. 74037 is provided at the lower end of the flexible tongues with external engagement cams which, with the adapter sleeve inserted, engage below the lower edge of the hole in the skull bone and thus fix the adapter sleeve in position. In order to adjust the reference depth, an intermediate ring can be slipped over the adapter sleeve from the free ends of the flexible tongues, which ring rests on the shoulder in the bore when the adapter sleeve has been inserted into the hole in the skull bone. Although reproducible stepped bores can generally be made with surgical stepped trephines, clinical tests have shown that the step height can nevertheless vary. For example, at a mean step height of 2.6 mm, variations between 1.7 mm and 3.8 mm step height were found. Since, however, the membrane of the pressure sensor must be adjusted to a precisely defined depth position, the variations in the step height are compensated in the adapter assembly of EPA 74037 by means of variously sized intermediate rings.
According to EPA No. 74037, the pressure sensor is screwed into the adapter sleeve which is provided for this purpose with an internal thread in its upper region. A certain depth adjustment is also achievable via this thread. In addition, the internal thread in the upper region of the expanding sleeve serves for anchoring an insertion and replacement tool. However, injuries on removal or replacement of the sensor are possible even with the adapter of EPA No. 74037, especially after a prolonged measurement time, since, as mentioned therein, sticking can occur in the lower region of the adapter sleeve and this inevitably causes injuries when the adapter sleeve is pulled out, even though these injuries are less serious than in the case where the adapter sleeve is screwed into the skull bone.
A particular problem associated with the pressure sensor adapter assembly, according to EPA No. 74037, is due to the fact that the flexible tongues of the adapter sleeve, initially only loosely inserted into the stepped bore in the skull bone, are not forced axially outwards, so that the engagement cams at the lower ends of the flexible tongues cannot engage below the lower peripheral edge of the drilled hole, until the pressure sensor is inserted into the adapter sleeve. It is then unavoidable that a certain pressure is transmitted to the measurement membrane of the sensor, particularly in the edge region thereof. This is the case especially if the pressure sensor is inserted even with only a slight tilt into the adapter sleeve. For an accurate pressure measurement at the dura, it is absolutely necessary, however, that no other pressure forces whatsoever act on the measurement membrane of the pressure sensor.
Another problem associated with the pressure sensor adapter assembly of EPA 74307 arises from the fact that the reference depth also is reproducible only in a relatively complicated or inaccurate manner. Because of the holding and adjusting thread used in the adapter sleeve, the sensor must also be constructed with relatively sharp edges and must be rotated on insertion, which, in turn involves the risk of uncontrollable pressure forces acting on the underside, that is to say in the region of the membrane edge, so that the measured result can be inaccurate. Even though EPA No. 74307 suggests the use of a bayonet closure as an alternative, this has the consequence that adjusting means of another type must then be used. Tests carried out over a prolonged period have shown that, when a bayonet closure is used, it is absolutely necessary to check repeatedly whether the closure has adequately snapped in. The feared tilting, already mentioned, can then again easily lead to inaccuracies in the measured result. By contrast, if the expanding sleeve is held loosely, it happens quite frequently that the sleeve is seated too loosely and also turns, instead of the sensor really snapping into the bayonet closure.